The present invention relates to cardiac pacemakers of the physiological type, particularly those which operate in the DDD and DVI modes.
Pacing modalities of cardiac pacemakers are designated by a three-letter code which was authorized by the Inter-Society Commission for Heart Disease Resources as a standardized means of identifying the operation and mode of an implantable pulse generator. The first letter of this Code indicates the chamber which is paced. A capital V indicates the ventricle chamber, a capital A the atrium, and a capital D indicates that both chambers may be placed. The second letter employs the same designations and indicates whether the ventricle or the atrium alone is sensed, or whether both are sensed. The third letter of the code indicates the mode of response. A capital I indicates an inhibited mode, a capital T a triggered mode, and a capital D a "double mode." The double mode is more particularly defined as an atrial-triggered, ventricular-inhibited mode of operation.
In recent years the vast majority of electronic pacemakers which were implanted in cardiac patients were of the ventricular demand type. The three-letter pacing modality code for this type of pacemaker is VVI, which indicates that the ventricle is both paced and sensed, and that the pacing output of the implanted pacemaker is inhibited when a natural ventricular pulse is sensed by the artificial pacemaker. With the ventricular demand pacemaker, only a single lead is inserted in the ventricle and the pulses that occur naturally in the atrium are, therefore, not sensed. Modern physiological cardiac pacemakers are used with leads inserted into both the atrial and the ventricular chambers of the heart and rely on synchronization of timing cycles so that ventricular pulses occur at an appropriate time after the occurrence of atrial pulses. The proper timing between the atrial and the ventricular pulses is called A-V synchrony. By maintenance of proper A-V synchrony, an effective increase in cardiac output of up to 30 percent may be obtained. In other words, physiologic pacing can restore, to a large degree, the contribution of the atrium chamber, thereby achieving substantial hemodynamic improvement over VVI pacing and, in addition, a measure of control of cardiac arrhythmias can be obtained by these modern pacing modes.
There are three types of pacing modalities which are characterized as the physiological. The first is the atrail, or P-wave synchronous mode which is designated as the VDD mode. This means that both chambers are sensed, but only the ventricle chamber is paced.
The next physiological pacing mode is the A-V sequential pacing mode which is designated by the letters DVI. As indicated, this mode is one in which both chambers may be paced, but sensing occurs only in the ventricle chamber, and the occurrence of a natural pulse in the ventricle chamber during a predetermined timing cycle will inhibit pacing of the ventricle during that timing cycle.
The third physiological pacing modality is called the universal mode, or the DDD mode. This indicates that both chambers may be either sensed or paced, and the mode of response is atrial-triggered, ventricular-inhibited.
In prior physiologic cardiac pacemakers, the sensing of a natural atrial pulse, or P-wave, or the occurrence of a produced atrial pacing pulses serves to initiate the start of a timing period called the A-V interval. During the A-V interval, the ventricular sense amplifier is capable of sensing a naturally occurring ventricular pulse, or R-wave. In these prior cardiac pacemakers, if an R-wave is sensed during the A-V interval, a second time period called the V-A interval is immediately initiated, at the end of which an atrial pulse is generated. If the A-V interval times out without the sensing of an R-wave, however, the V-A interval begins at the end of the A-V interval.
The V-A interval was thus a fixed time interval in prior physiologic pacemakers, and the resulting A-A interval, which is total time period from the atrial pacing pulse that initiated the A-V interval and the end of the V-A interval, was variable when the V-A interval was initiated by an R-wave. The A-A interval, however, was fixed when it was started by a ventricular pacing pulse. (The interval between the start of the A-V interval and the occurrence of a R-wave is defined as the A-R interval). In cases where the patient's natural A-R interval became much less than the programmed A-V interval, the resulting pacing rate becomes much faster than intended, and this resulted in a serious potential hazard to the patient. The present invention resolves this serious problem found in prior physiologic cardiac pacemakers.